remove buggy cookie path detection

[Rich-Harris]

closes #9130

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[dummdidumm]

I think the problem is not with how we set the incoming cookie, because through cookie.set we can track the path correctly. I think in the case of tests the problem is that we basically track the existing cookie paths in a singleton, which likely spans all Playwright workers. I'm not sure how others run into this, but given the comments in the issue it seems to happen, so I'm ok with removing some these checks.

(I would also remove the comment in line 126/127 because it references the now removed warning from the delete branch)